4k mode interleaver with odd interleaving only and per ofdm symbol permutation code change in a digital video broadcasting (dvb) standard

ABSTRACT

A data processing apparatus maps input symbols to be communicated onto a predetermined number of sub-carrier signals of an Orthogonal Frequency Division Multiplexed (OFDM) symbol. The data processor includes an interleaver memory which reads-in the predetermined number of data symbols for mapping onto the OFDM sub-carrier signals. The interleaver memory reads-out the data symbols on to the OFDM sub-carriers to effect the mapping, the read-out being in a different order than the read-in, the order being determined from a set of addresses, with the effect that the data symbols are interleaved on to the sub-carrier signals. The set of addresses are generated from an address generator which comprises a linear feedback shift register and a permutation circuit. The linear feedback shift register has eleven register stages with a generator polynomial for the linear feedback shift register of R i ′[10]=R i-1 ′[0]⊕R i-1 ′[2], and the permutation code forms, with an additional bit, a twelve bit address.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional application of U.S. applicationSer. No. 12/257,010, filed Oct. 23, 2008, the entire contents of whichare incorporated herein by reference. This application also claimspriority to United Kingdom Applications Nos. 0721269.9, filed Oct. 30,2007; 0722645.9, filed Nov. 19, 2007; and 0722728.3, filed Nov. 20,2007.

FIELD OF INVENTION

The present invention relates to data processing apparatus operable tomap input symbols onto sub-carrier signals of Orthogonal FrequencyDivision Multiplexed (OFDM) symbols.

The present invention also relates to data processing apparatus operableto map symbols received from a predetermined number of sub-carriersignals of OFDM symbols into an output symbol stream.

Embodiments of the present invention can provide an OFDMtransmitter/receiver.

BACKGROUND OF THE INVENTION

The Digital Video Broadcasting-Terrestrial standard (DVB-T) utilisesOrthogonal Frequency Division Multiplexing (OFDM) to communicate datarepresenting video images and sound to receivers via a broadcast radiocommunications signal. There are known to be two modes for the DVB-Tstandard which are known as the 2k and the 8k mode. The 2k mode provides2048 sub-carriers whereas the 8k mode provides 8192 sub-carriers.Similarly for the Digital Video Broadcasting-Handheld standard (DVB-H) a4k mode has been provided, in which the number of sub-carriers is 4096.

Error correction coding schemes such as Low Density ParityCheck/Bose-Chaudhuri-Hocquengham (LDPC/BCH) coding, which have beenproposed for DVB-T2 perform better when noise and degradation of thesymbol values resulting from communication is un-correlated. Terrestrialbroadcast channels may suffer from correlated fading in both the timeand the frequency domains. As such, by separating encoded symbols on todifferent sub-carrier signals of the OFDM symbol by as much as possible,the performance of error correction coding schemes can be increased.Accordingly, in order to improve the integrity of data communicatedusing DVB-T or DVB-H, a symbol interleaver is provided in order tointerleave input data symbols as these symbols are mapped onto thesub-carrier signals of an OFDM symbol. Such a symbol interleavercomprises an interleaver memory and an address generator. Theinterleaver is arranged to read-into the interleaver memory the datasymbols for mapping onto the OFDM sub-carrier signals, and to read-outof the memory the data symbols for the OFDM sub-carriers, the read-outbeing in a different order than the read-in, the order being determinedfrom a set of addresses, which are generated by the address generator.For the 2k mode and the 8k mode an arrangement has been disclosed in theDVB-T standard for generating the addresses to effect the mapping.Likewise for the 4k mode of DVB-H standard, an arrangement forgenerating addresses for the mapping has been provided and an addressgenerator for implementing this mapping is disclosed in European Patentapplication 04251667.4. The address generator comprises a linear feedback shift register which is operable to generate a pseudo random bitsequence and a permutation circuit. The permutation circuit permutes theorder of the content of the linear feed back shift register in order togenerate an address. The address provides an indication of a memorylocation of the interleaver memory for writing the input data symbolinto or reading the input data symbol out from the interleaver memoryfor mapping onto one of the sub-carrier signal of the OFDM symbol.Similarly, an address generator in the receiver is arranged to generateaddresses of the interleaver memory for writing the received datasymbols into or reading the data symbols out from the interleaver memoryto form an output data stream.

In accordance with a further development of the Digital VideoBroadcasting-Terrestrial standard, known as DVB-T2, there is a desire toimprove the communication of data, and more particularly to provide animproved arrangement for interleaving the data symbols onto thesub-carrier signals of OFDM symbols.

SUMMARY OF INVENTION

According to an aspect of the present invention there is provided a dataprocessing apparatus operable to map input data symbols to becommunicated onto a predetermined number of sub-carrier signals of anOrthogonal Frequency Division Multiplexed (OFDM) symbols. The dataprocessing apparatus comprises an interleaver operable to read-into amemory the predetermined number of data symbols for mapping onto theOFDM sub-carrier signals, and to read-out of the memory the data symbolsfor the OFDM sub-carriers to effect the mapping, the read-out being in adifferent order than the read-in, the order being determined from a setof addresses, with the effect that the data symbols are interleaved onthe sub-carrier signals. The data processing apparatus includes anaddress generator operable to generate the set of addresses, an addressbeing generated for each of the input data symbols for mapping the inputdata symbols onto the sub-carrier signals. The address generatorcomprises a linear feedback shift register including a predeterminednumber of register stages, which are operable to generate apseudo-random bit sequence in accordance with a generator polynomial, apermutation circuit arranged to receive the content of the shiftregister stages and to permute the order of the bits present in theregister stages in accordance with a permutation code to form an addressof one of the OFDM sub-carriers, and a control unit operable incombination with an address check circuit to re-generate an address whena generated address exceeds a predetermined maximum valid address. Thepredetermined maximum valid address is approximately four thousand, thelinear feedback shift register has eleven register stages with agenerator polynomial for the linear feedback shift register ofR_(i)′[10]=R_(i-1)′[0]⊕R_(i-1)′[2], and the permutation order forms,with an additional bit, a twelve bit address. The data processingapparatus is characterised in that the permutation circuit is arrangedto change the permutation code, which permutes the order of the bits ofthe register stages to form the set of addresses from one OFDM symbol toanother.

Embodiments of the present invention can provide a data processingapparatus operable as a symbol interleaver for mapping data symbols tobe communicated on an OFDM symbol, having substantially four thousandsub-carrier signals, which can provide an improvement in the integrityof the communicated data. The improvement is provided as a result of achange of the permutation code, which is used to change the order of thebits in the feed back shift register, from one OFDM symbol to another.For example, the permutation code used may be one of a sequence ofdifferent permutation codes which is cycled through, for each of aplurality of OFDM symbols. As a result, an improvement is provided inreducing a possibility that successive or data bits which are close inorder in an input data stream are mapped onto the same sub-carrier of anOFDM symbol, so that the error correction encoding can work moreefficiently.

In one embodiment the number of sub-carrier signals maybe a valuesubstantially between two thousand and four thousand and ninety six.Furthermore, the OFDM symbol may include pilot sub-carriers, which arearranged to carry known symbols, and the predetermined maximum validaddress may depend on a number of the pilot sub-carrier symbols presentin the OFDM symbol. As such the 4k mode can be provided with anefficient symbol interleaver, for example for a DVB standard, such asDVB-T2, DVB-T or DVB-H.

In one example the sequence of different permutation codes forms thetwelve bit address R_(i)[n] for the i-th data symbol from the bitpresent in the n-th register stage R_(i)′[n] in accordance with thepermutation code defined by the table:

R_(i)′[n] for n = 10 9 8 7 6 5 4 3 2 1 0 R_(i)[n] for n = 7 10 5 8 1 2 49 0 3 6

Although the sequence of permutation codes can include any number ofpermutation codes, in one example there are two permutation codes. Inone example, the two permutation codes are:

R_(i)′[n] for n = 10 9 8 7 6 5 4 3 2 1 0 R_(i)[n] for n = 7 10 5 8 1 2 49 0 3 6

and

R_(i)′[n] for n = 10 9 8 7 6 5 4 3 2 1 0 R_(i)[n] for n = 6 2 7 10 8 0 34 1 9 5

For example, the approximately four thousand sub-carriers may beprovided as one of a plurality of operating modes, the approximatelyfour thousand sub-carriers providing half or less than half a maximumnumber of sub-carriers in the OFDM symbols of any of the operatingmodes. The input data symbols may be formed into or regarded as firstsets of input data symbols for mapping onto first OFDM symbols andsecond sets of input data symbols for mapping onto second OFDM symbols.The data processing apparatus may be operable to interleave the inputdata symbols from both first and second sets in accordance with an oddinterleaving process. The odd interleaving process includes writing thefirst sets of input data symbols into a first part of the interleavermemory in accordance with a sequential order of the first sets of inputdata symbols, reading out the first sets of input data symbols from thefirst part of the interleaver memory on to the sub-carrier signals ofthe first OFDM symbols in accordance with an order defined by one of thepermutation codes of the sequence, writing the second set of input datasymbols into a second part of the interleaver memory in accordance witha sequential order of the second sets of input data symbols, and readingout the second sets of input data symbols from the second part of theinterleaver memory on to the sub-carrier signals of the second OFDMsymbols in accordance with an order defined by another of thepermutation codes of the sequence.

The first OFDM symbols may be odd OFDM symbols, and the second OFDMsymbols may be even OFDM symbols.

In some conventional OFDM transmitters and receivers, which operate inaccordance with the 2k or 8k modes for DVB-T and the 4k mode for DVB-H,two symbol interleaving processes are used in the transmitter and thereceiver; one for even OFMD symbols and one for odd OFMD symbols.However, analysis has shown that the interleaving schemes designed forthe 2k and 8k symbol interleavers for DVB-T and the 4k symbolinterleaver for DVB-H work better for odd symbols than for even symbols.Embodiments of the present invention are arranged so that only the oddsymbol interleaving process is used unless the transmitter/receiver isin the mode with the maximum number of sub-carriers. Therefore, when thenumber data symbols which can be carried by the sub-carriers of an OFDMsymbol in one of the plurality of operating modes is less than half ofthe number of data symbols, which can be carried in an operating modewhich proves the most number of data bearing sub-carrier signals perOFDM symbol, then an interleaver of the transmitter and the receiver ofthe OFDM symbols is arranged to interleaver the data symbols of both thefirst and second sets using the odd interleaving process. Since theinterleaver is interleaving the data symbols of both the first andsecond sets of data symbols onto the OFDM symbols using the oddinterleaving process, the interleaver uses different parts of theinterleaver memory to write in and read out the data symbols. Thus,compared with the example in which the interleaver is using the oddinterleaving process and the even interleaving process to interleave thefirst and second sets of data symbols onto successive first and secondOFDM symbols, which utilises the available memory, the amount of memorycapacity used is twice the number of data symbols which can be carriedby an OFDM symbol for the odd only interleaving. This is compared with amemory requirement of one times the number of data symbols, which can becarried in an OFDM symbol in the mode with the most number of datasymbols per OFDM symbol using both the odd and even interleavingprocesses. However, the number of sub-carriers per OFDM symbol for thismaximum operating mode is twice the capacity of the next largest numberof sub-carriers per OFDM symbol for any other operating mode with thenext largest number of sub-carriers per OFDM symbol.

According to some examples therefore, a minimum size of the interleavermemory can be provided in accordance with the maximum number of inputdata symbols which can be carried on the sub-carriers of the OFDMsymbols which are available to carry the input data symbols in any ofthe operating modes.

In some embodiments the operating mode which provides the maximum numberof sub-carriers per OFDM symbol is a 32k mode. The other modes mayinclude one or more of 1k, 2k, 4k, 8k and 16k modes. Thus, as will beappreciated from the above explanation, in the 32k mode the odd and eveninterleaving processes are used to interleave the data symbols, so thatthe size of the interleaver memory can be just enough to account for 32kdata symbols. However, for the 16k mode and any of the other modes, thenthe odd interleaving process only is used, so that with the 16k mode anequivalent memory size of 32K symbols is required, with the 4k mode anequivalent memory size of 8k symbols is required, and with the 2k modean equivalent memory size of 4k symbols is required.

Various aspects and features of the present invention are defined in theappended claims. Further aspects of the present invention include amethod of mapping input symbols to be communicated onto a predeterminednumber of sub-carrier signals of an Orthogonal Frequency DivisionMultiplexed (OFDM) symbol, as well as a transmitter.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present invention will now be described by way ofexample only with reference to the accompanying drawings, wherein likeparts are provided with corresponding reference numerals, and in which:

FIG. 1 is a schematic block diagram of a Coded OFDM transmitter whichmay be used, for example, with the DVB-T2 standard;

FIG. 2 is a schematic block diagram of parts of the transmitter shown inFIG. 1 in which a symbol mapper and a frame builder illustrate theoperation of an interleaver;

FIG. 3 is a schematic block diagram of the symbol interleaver shown inFIG. 2;

FIG. 4 is a schematic block diagram of an interleaver memory shown inFIG. 3 and the corresponding symbol de-interleaver in the receiver;

FIG. 5 is a schematic block diagram of an address generator shown inFIG. 3 for the 4k mode;

FIG. 6 is a schematic block diagram of a Coded OFDM receiver which maybe used, for example, with the DVB-T2 standard;

FIG. 7 is a schematic block diagram of a symbol de-interleaver whichappears in FIG. 6;

FIG. 8( a) is diagram illustrating results for an interleaver for evenOFDM symbols and FIG. 8( b) is a diagram illustrating results for oddOFDM symbols;

FIGS. 8( a) and 8(b) show plots of the distance at the interleaveroutput of sub-carriers that were adjacent at the interleaver input;

FIG. 9 provides a schematic block diagram of the symbol interleavershown in FIG. 3, illustrating an operating mode in which interleaving isperformed in accordance with an odd interleaving mode only; and

FIG. 10 provides a schematic block diagram of the symbol de-interleavershown in FIG. 7, illustrating the operating mode in which interleavingis performed in accordance with the odd interleaving mode only.

DESCRIPTION OF PREFERRED EMBODIMENTS

The following description is provided to illustrate the operation of asymbol interleaver in accordance with the present technique, although itwill be appreciated that the symbol interleaver can be used with othermodes, other DVB standards and other OFDM systems.

FIG. 1 provides an example block diagram of a Coded OFDM transmitterwhich may be used for example to transmit video images and audio signalsin accordance with the DVB-T2 standard. In FIG. 1 a program sourcegenerates data to be transmitted by the Coded Orthogonal FrequencyDivision Multiplexing (COFDM) transmitter. A video coder 2, and audiocoder 4 and a data coder 6 generate video, audio and other data to betransmitted which are fed to a program multiplexer 10. The output of theprogram multiplexer 10 forms a multiplexed stream with other informationrequired to communicate the video, audio and other data. The multiplexer10 provides a stream on a connecting channel 12. There may be many suchmultiplexed streams which are fed into different branches A, B etc. Forsimplicity, only branch A will be described.

As shown in FIG. 1 a COFDM transmitter receives the stream at amultiplexer adaptation and energy dispersal block 22. The multiplexeradaptation and energy dispersal block 22 randomises the data and feedsthe appropriate data to a forward error correction encoder 24 whichperforms error correction encoding of the stream. A bit interleaver 26is provided to interleave the encoded data bits which for the example ofDVB-T2 is the LDCP/BCH encoder output. The output from the bitinterleaver 26 is fed to a bit into constellation mapper 28, which mapsgroups of bits onto a constellation point, which is to be used forconveying the encoded data bits. The outputs from the bit intoconstellation mapper 28 are constellation point labels that representreal and imaginary components. The constellation point labels representdata symbols formed from two or more bits depending on the modulationscheme used. These will be referred to as data cells. These data cellsare passed through a time-interleaver 30 whose effect is to interleaverdata cells resulting from multiple LDPC code words.

The data cells are received by a frame builder 32, with data cellsproduced by branch B etc in FIG. 1, via other channels 31. The framebuilder 32 then forms many data cells into sequences to be conveyed onCOFDM symbols, where a COFDM symbol comprises a number of data cells,each data cell being mapped onto one of the sub-carriers. The number ofsub-carriers will depend on the mode of operation of the system, whichmay include one of 1k, 2k, 4k, 8k, 16k or 32k, each of which provides adifferent number of sub-carriers according, for example to the followingtable:

Number of Sub-carriers Adapted from DVB-T/H Mode Sub-carriers 1k 756 2k1512 4k 3024 8k 6048 16k  12096 32k  24192

Thus in one example, the number of sub-carriers for the 4k mode is threethousand and twenty four. For the DVB-T2 system, the number ofsub-carriers per OFDM symbol can vary depending upon the number of pilotand other reserved carriers. Thus, in DVB-T2, unlike in DVB-T, thenumber of sub-carriers for carrying data is not fixed. Broadcasters canselect one of the operating modes from 1k, 2k, 4k, 8k, 16k, 32k eachproviding a range of sub-carriers for data per OFDM symbol, the maximumavailable for each of these modes being 1024, 2048, 4096, 8192, 16384,32768 respectively. In DVB-T2 a physical layer frame is composed of manyOFDM symbols. Typically the frame starts with one or more preamble or P2OFDM symbols, which are then followed by a number payload carrying OFDMsymbols. The end of the physical layer frame is marked by a frameclosing symbols. For each operating mode, the number of sub-carriers maybe different for each type of symbol. Furthermore, this may vary foreach according to whether bandwidth extension is selected, whether tonereservation is enabled and according to which pilot sub-carrier patternhas been selected. As such a generalisation to a specific number ofsub-carriers per OFDM symbol is difficult. However, the frequencyinterleaver for each mode can interleave any symbol whose number ofsub-carriers is smaller than or the same as the maximum available numberof sub-carriers for the given mode. For example, in the 1k mode, theinterleaver would work for symbols with the number of sub-carriers beingless than or equal to 1024 and for 16k mode, with the number ofsub-carriers being less than or equal to 16384.

The sequence of data cells to be carried in each COFDM symbol is thenpassed to the symbol interleaver 33. The COFDM symbol is then generatedby a COFDM symbol builder block 37 which introduces pilot andsynchronising signals fed from a pilot and embedded signal former 36. AnOFDM modulator 38 then forms the OFDM symbol in the time domain which isfed to a guard insertion processor 40 for generating a guard intervalbetween symbols, and then to a digital to analogue convertor 42 andfinally to an RF amplifier within an RF front end 44 for eventualbroadcast by the COFDM transmitter from an antenna 46.

As explained above, the present invention provides a facility forproviding a quasi-optimal mapping of the data symbols onto the OFDMsub-carrier signals. According to the example technique the symbolinterleaver is provided to effect the optimal mapping of input datasymbols onto COFDM sub-carrier signals in accordance with a permutationcode and generator polynomial, which has been verified by simulationanalysis.

As shown in FIG. 2 a more detailed example illustration of the bit tosymbol constellation mapper 28 and the frame builder 32 is provided toillustrate an example embodiment of the present technique. Data bitsreceived from the bit interleaver 26 via a channel 62 are grouped intosets of bits to be mapped onto a data cell, in accordance with a numberof bits per symbol provided by the modulation scheme. The groups ofbits, which forms a data word, are fed in parallel via data channels 64the a mapping processor 66. The mapping processor 66 then selects one ofthe data symbols, in accordance with a pre-assigned mapping. Theconstellation point, is represented by a real and an imaginary componentthat is provided to the output channel 29 as one of a set of inputs tothe frame builder 32.

The frame builder 32 receives the data cells from the bit toconstellation mapper 28 through channel 29, together with data cellsfrom the other channels 31. After building a frame of many COFDM cellsequences, the cells of each COFDM symbol are then written into aninterleaver memory 100 and read out of the interleaver memory 100 inaccordance with write addresses and read addresses generated by anaddress generator 102. According to the write-in and read-out order,interleaving of the data cells is achieved, by generating appropriateaddresses. The operation of the address generator 102 and theinterleaver memory 100 will be described in more detail shortly withreference to FIGS. 3, 4 and 5. The interleaved data cells are thencombined with pilot and synchronisation symbols received from the pilotand embedded signalling former 36 into an OFDM symbol builder 37, toform the COFDM symbol, which is fed to the OFDM modulator 38 asexplained above.

Interleaver

FIG. 3 provides an example of parts of the symbol interleaver 33, whichillustrates the present technique for interleaving symbols. In FIG. 3the input data cells from the frame builder 32 are written into theinterleaver memory 100. The data cells are written into the interleavermemory 100 according to a write address fed from the address generator102 on channel 104, and read out from the interleaver memory 100according to a read address fed from the address generator 102 on achannel 106. The address generator 102 generates the write address andthe read address as explained below, depending on whether the COFDMsymbol is odd or even, which is identified from a signal fed from achannel 108, and depending on a selected mode, which is identified froma signal fed from a channel 110. As explained, the mode can be one of a1k mode, 2k mode, 4k mode, 8k mode, 16k mode or a 32k mode. As explainedbelow, the write address and the read address are generated differentlyfor odd and even symbols as explained with reference to FIG. 4, whichprovides an example implementation of the interleaver memory 100.

In the example shown in FIG. 4, the interleaver memory is shown tocomprise an upper part 100 illustrating the operation of the interleavermemory in the transmitter and a lower part 340, which illustrates theoperation of the de-interleaver memory in the receiver. The interleaver100 and the de-interleaver 340 are shown together in FIG. 4 in order tofacilitate understanding of their operation. As shown in FIG. 4 arepresentation of the communication between the interleaver 100 and thede-interleaver 340 via other devices and via a transmission channel hasbeen simplified and represented as a section 140 between the interleaver100 and the de-interleaver 340. The operation of the interleaver 100 isdescribed in the following paragraphs:

Although FIG. 4 provides an illustration of only four input data cellsonto an example of four sub-carrier signals of a COFDM symbol, it willbe appreciated that the technique illustrated in FIG. 4 can be extendedto a larger number of sub-carriers such as 756 for the 1k mode 1512 forthe 2k mode, 3024 for the 4k mode and 6048 for the 8k mode, 12096 forthe 16k mode and 24192 for the 32k mode.

The input and output addressing of the interleaver memory 100 shown inFIG. 4 is shown for odd and even symbols. For an even COFDM symbol thedata cells are taken from the input channel and written into theinterleaver memory 124.1 in accordance with a sequence of addresses 120generated for each COFDM symbol by the address generator 102. The writeaddresses are applied for the even symbol so that as illustratedinterleaving is effected by the shuffling of the write-in addresses.Therefore, for each interleaved symbol y(h(q))=y′(q).

For odd symbols the same interleaver memory 124.2 is used. However, asshown in FIG. 4 for the odd symbol the write-in order 132 is in the sameaddress sequence used to read out the previous even symbol 126. Thisfeature allows the odd and even symbol interleaver implementations toonly use one interleaver memory 100 provided the read-out operation fora given address is performed before the write-in operation. The datacells written into the interleaver memory 124.2 during odd symbols arethen read out in a sequence 134 generated by the address generator 102for the next even COFDM symbol and so on. Thus only one address isgenerated per symbol, with the read-in and write-out for the odd/evenCOFDM symbol being performed contemporaneously.

In summary, as represented in FIG. 4, once the set of addresses H(q) hasbeen calculated for all active sub-carriers, the input vector Y′=(y′₀,y′₁, y′₂, . . . y′_(Nmax=1)) is processed to produce the interleavedvector Y=(y₀, y₁, y₂, . . . y_(Nmax-1)) defined by:

y_(H(q))=y′_(q) for even symbols for q=0, . . . , N_(max)−1

y_(q)=y′_(H(q)) for odd symbols for q=0, . . . , N_(max)−1

In other words, for even OFDM symbols the input words are written in apermutated way into a memory and read back in a sequential way, whereasfor odd symbols, they are written sequentially and read back permutated.In the above case, the permutation H(q) is defined by the followingtable:

TABLE 1 permutation for simple case where Nmax = 4 q 0 1 2 3 H(q) 1 3 02

As shown in FIG. 4, the de-interleaver 340 operates to reverse theinterleaving applied by the interleaver 100, by applying the same set ofaddresses as generated by an equivalent address generator, but applyingthe write-in and read-out addresses in reverse. As such, for evensymbols, the write-in addresses 342 are in sequential order, whereas theread out address 344 are provided by the address generator.Correspondingly, for the odd symbols, the write-in order 346 isdetermined from the set of addresses generated by the address generator,whereas read out 348 is in sequential order.

Address Generation for the 4k Mode

A schematic block diagram of the algorithm used to generate thepermutation function H(q) is represented in FIG. 5 for the 4K mode. InFIG. 5 a linear feed back shift register is formed by twelve shiftregister stages 200, in order to generate an address between 0 and fourthousand and ninety five, and an xor-gate 202 which is connected to thestages of the shift register 200 in accordance with a generatorpolynomial. Therefore, in accordance with the content of the shiftregister 200 a next bit of the shift register is provided from theoutput of the xor-gate 202 by xoring the content of shift register R[0]and register stage R[2]. According to the generator polynomial a pseudorandom bit sequence is generated from the content of the shift register200. However, in order to generate an address for the 4k mode asillustrated, a permutation circuit 210 is provided which effectivelypermutes the order of the bits within the shift register 200 from anorder R′_(i)[n] to an order R_(i)[n] at the output of the permutationcircuit 210. Eleven bits from the output of the permutation circuit 210are then fed on a connecting channel 212 to which is added a mostsignificant bit via a channel 214 which is provided by a toggle circuit218. A twelve bit address is therefore generated on channel 212.However, in order to ensure the authenticity of an address, an addresscheck circuit 216 analyses the generated address to determine whether itexceeds the maximum number of sub-carrier signals. If it does then acontrol signal is generated and fed via a connecting channel 220 to acontrol unit 224. If the generated address exceeds the maximum number ofcarrier signals then this address is rejected and a new addressregenerated for the particular symbol.

In summary an (N_(r)−1) bit word R′_(i) is defined, with N_(r)=log₂M_(max), where M_(max)=4096 in the 4K mode, using a LFSR (LinearFeedback Shift Register).

The polynomials used to generate this sequence are as follows:

4K mode: R_(i)′[10]=R_(i-1)′[0]⊕R_(i-1)′[2]

where i varies from 0 to M_(max)−1

Once one R′_(i), word has been generated, it goes through a permutationto produce another (N_(r)−1) bit word called R_(i). R_(i) is derivedfrom R′_(i) by the bit permutations given in the table below:

TABLE Bit permutation for the 4K mode R_(i)′[n] for n = 10 9 8 7 6 5 4 32 1 0 R_(i)[n] for n = 7 10 5 8 1 2 4 9 0 3 6

As an example, for the permutation code above this means that for mode4K, the bit number 10 of R′_(i) is sent in bit position number 7 ofR_(i).

The address H(q) is then derived from R_(i) through the followingequation:

${H(q)} = {{\left( {i\; {mod}\; 2} \right) \cdot 2^{N_{r} - 1}} + {\sum\limits_{j = 0}^{N_{r} - 2}{{R_{i}(j)} \cdot 2^{j}}}}$

The (i mod 2)·2^(N) ^(r) ⁻¹ part of the above equation is represented inFIG. 5 by the toggle block T 218.

An address check is then performed on H(q) to verify that the generatedaddress is within the range of acceptable addresses: if (H(q)<N_(max)),where, in one example N_(max)=3024 in the 4K mode, then the address isvalid. If the address is not valid, the control unit is informed and itwill try to generate a new H(q) by incrementing the index i.

The role of the toggle block is to make sure that we do not generate anaddress exceeding N_(max) twice in a row. In effect, if an exceedingvalue was generated, this means that the MSB (i.e. the toggle bit) ofthe address H(q) was one. So the next value generated will have a MSBset to zero, insuring to produce a valid address.

The following equations sum up the overall behaviour and help tounderstand the loop structure of this algorithm:

  q = 0;   for  (i = 0; i < M_(max); i = i + 1)$\left\{ {{{H(q)} = {{\left( {i\; {mod}\; 2} \right) \cdot 2^{N_{r} - 1}} + {\sum\limits_{j = 0}^{N_{r} - 2}{{R_{i}(j)} \cdot 2^{j}}}}};{{{if}\mspace{14mu} \left( {{H(q)} < N_{\max}} \right)q} = {q + 1}};} \right\}$

As will be explained shortly, in one example of the address generator,the above mentioned permutation code is used for generating addressesfor all OFDM symbols. In another example, the permutation codes may bechanged between symbols, with the effect that a set of permutation codesare cycled through for successive OFMD symbols. To this end, the controllines 108, 110 providing an indication as to whether the OFDM symbol isodd or even and the current mode are used to select the permutationcode. This example mode in which a plurality of permutation codes arecycled through is particularly appropriate for the example in which theodd interleaver only is used, which will be explained later. A signalindicating that a different permutation code should be used is providedvia a control channel. In one example the possible permutation codes arepre-stored in the permutation code circuit 210. In another example, thecontrol unit 224 supplies the new permutation code to be used for anOFDM symbol.

Receiver

FIG. 6 provides an example illustration of a receiver which may be usedwith the present technique. As shown in FIG. 6, a COFDM signal isreceived by an antenna 300 and detected by a tuner 302 and convertedinto a digital form by an analogue-to-digital converter 304. A guardinterval removal processor 306 removes the guard interval from areceived COFDM symbol, before the data is recovered from the COFDMsymbol using a Fast Fourier Transform (FFT) processor 308 in combinationwith a channel estimator and correction 310 in co-operation with aembedded-signalling decoding unit 311, in accordance with knowntechniques. The demodulated data is recovered from a mapper 312 and fedto a symbol de-interleaver 314, which operates to effect the reversemapping of the received data symbol to re-generate an output data streamwith the data de-interleaved.

The symbol de-interleaver 314 is formed from a data processing apparatusas shown in FIG. 7 with an interleaver memory 540 and an addressgenerator 542. The interleaver memory is as shown in FIG. 4 and operatesas already explained above to effect de-interleaving by utilising setsof addresses generated by the address generator 542. The addressgenerator 542 is formed as shown in FIG. 7 and is arranged to generatecorresponding addresses to map the data symbols recovered from eachCOFDM sub-carrier signals into an output data stream.

The remaining parts of the COFDM receiver shown in FIG. 6, including BitDe-Interleaver 316, are provided to effect error correction decoding 318to correct errors and recover an estimate of the source data.

One advantage provided by the present technique for both the receiverand the transmitter is that a symbol interleaver and a symbolde-interleaver operating in the receivers and transmitters can beswitched between the 1k, 2k, 4k, 8k, 16k and the 32k mode by changingthe generator polynomials and the permutation order. Hence the addressgenerator 542 shown in FIG. 7 includes an input 544, providing anindication of the mode as well as an input 546 indicating whether thereare odd/even COFDM symbols. A flexible implementation is therebyprovided because a symbol interleaver and de-interleaver can be formedas shown in FIGS. 3 and 7, with an address generator as illustrated inFIG. 5. The address generator can therefore be adapted to the differentmodes by changing to the generator polynomials and the permutationorders indicated for each of the modes. For example, this can beeffected using a software change. Alternatively, in other embodiments,an embedded signal indicating the mode of the DVB-T2 transmission can bedetected in the receiver in the embedded-signalling processing unit 311and used to configure automatically the symbol de-interleaver inaccordance with the detected mode.

Optimal Use of Odd Interleavers

As shown in FIG. 4, two symbol interleaving processes, one for evenCOFDM symbols and one for odd COFDM symbols allows the amount of memoryused during interleaving to be reduced. In the example shown in FIG. 4,the write in order for the odd symbol is the same as the read out orderfor the even symbol therefore, while an odd symbol is being read fromthe memory, an even symbol can be written to the location just readfrom; subsequently, when that even symbol is read from the memory, thefollowing odd symbol can be written to the location just read from.

The selection of the polynomial generator and the permutation codesexplained above have been identified following simulation analysis ofthe relative performance of the interleaver. The relative performance ofthe interleaver has been evaluated using a relative ability of theinterleaver to separate successive symbols or an “interleaving quality”.The relative measure of the interleaver quality is determined bydefining a distance D (in number of sub-carriers). A criterion C ischosen to identify a number of sub-carriers that are at distance ≦D atthe output of the interleaver that were at distance ≦D at the input ofthe interleaver, the number of sub-carriers for each distance D thenbeing weighted with respect to the relative distance. The criterion C isevaluated for both odd and even COFDM symbols. Minimising C produces asuperior quality interleaver.

$C = {{\sum\limits_{1}^{d = D}{{N_{even}(d)}/d}} + {\sum\limits_{1}^{d = D}{{N_{odd}(d)}/d}}}$

where: N_(even)(d) and N_(odd)(d) are number of sub-carriers in an evenand odd symbol respectively at the output of the interleaver that remainwithin d sub-carrier spacing of each other.

As mentioned above, during an experimental analysis of the performanceof the interleavers (using criterion C as defined above) and for exampleshown in FIG. 8( a) and FIG. 8( b) it has been discovered that theinterleaving schemes designed for the 2k and 8k symbol interleavers forDVB-T and the 4k symbol interleaver for DVB-H work better for oddsymbols than even symbols. Thus from performance evaluation results ofthe interleavers, for example for the 16K, as illustrated by FIGS. 8( a)and 8(b) it has been revealed that the odd interleavers work better thanthe even interleavers. This can be seen by comparing FIG. 8( a) whichshows results for an interleaver for even symbols and FIG. 8( b)illustrating results for odd symbols: it can be seen that the averagedistance at the interleaver output of sub-carriers that were adjacent atthe interleaver input is greater for an interleaver for odd symbols thanan interleaver for even symbols.

As will be understood, the amount of interleaver memory required toimplement a symbol interleaver is dependent on the number of datasymbols to be mapped onto the COFDM carrier symbols. Thus a 16k modesymbol interleaver requires half the memory required to implement a 32kmode symbol interleaver and similarly, the amount of memory required toimplement an 8k symbol interleaver is half that required to implement a16k interleaver. Therefore a transmitter or receiver which is arrangedto implement a symbol interleaver of a mode, which sets the maximumnumber of data symbols which can be carried per OFDM symbol, then thatreceiver or transmitter will include sufficient memory to implement twoodd interleaving processes for any other mode, which provides half orsmaller than half the number of sub-carriers per OFDM symbol in thatgiven maximum mode. For example a receiver or transmitter including a32K interleaver will have enough memory to accommodate two 16K oddinterleaving processes each with their own 16K memory.

Therefore, in order to exploit the better performance of the oddinterleaving process, a symbol interleaver capable of accommodatingmultiple modulation modes can be arranged so that only an odd symbolinterleaving process is used if in a mode which comprises half or lessthan half of the number of sub-carriers in a maximum mode, whichrepresents the maximum number of sub-carriers per OFDM symbol. Thismaximum mode therefore sets the maximum memory size. For example, in atransmitter/receiver capable of the 32K mode, when operating in a modewith fewer carriers (i.e. 16K, 8K, 4K or 1K) then rather than employingseparate odd and even symbol interleaving processes, two oddinterleavers could be used.

An illustration is shown in FIG. 9 of an adaptation of the symbolinterleaver 33, which is shown in FIG. 3 when interleaving input datasymbols onto the sub-carriers of OFDM symbols in the odd interleavingmode only. The symbol interleaver 33.1 corresponds exactly to the symbolinterleaver 33 as shown in FIG. 3, except that the address generator 102is adapted to perform the odd interleaving process only. For the exampleshown in FIG. 9, the symbol interleaver 33.1 is operating in a modewhere the number of data symbols which can be carried per OFDM symbol isless than half of the maximum number which can be carried by an OFDMsymbol in an operating mode with the largest number of sub-carriers perOFDM symbol. As such, the symbol interleaver 33.1 has been arranged topartition the interleaver memory 100. For the present illustration shownin FIG. 9 the interleaver memory then 100 is divided into two parts 401,402. As an illustration of the symbol interleaver 33.1 operating in amode in which data symbols are mapped onto the OFDM symbols using theodd interleaving process, FIG. 9 provides an expanded view of each halfof the interleaver memory 401, 402. The expanded view provides anillustration of the odd interleaving mode as represented for thetransmitter side for four symbols A, B, C, D reproduced from FIG. 4.Thus as shown in FIG. 9, for successive sets of first and second datasymbols, the data symbols are written into the interleaver memory 401,402 in a sequential order and read out in a permuted order in accordancewith the addresses generated by the address generator 102 as previouslyexplained. Thus as illustrated in FIG. 9, since an odd interleavingprocess is being performed for successive sets of first and second setsof data symbols, the interleaver memory must be partitioned into twoparts. Symbols from a first set of data symbols are written into a firsthalf of the interleaver memory 401, and symbols from a second set ofdata symbols are written into a second part of the interleaver memory402. This is because the symbol interleaver is no longer able to reusethe same parts of the symbol interleaver memory as can be accommodatedwhen operating in an odd and even mode of interleaving.

A corresponding example of the interleaver in the receiver, whichappears in FIG. 7 but adapted to operate with an odd interleavingprocess only, is shown in FIG. 10. As shown in FIG. 10 the interleavermemory 540 is divided into two halves 410, 412 and the address generator542 is adapted to write data symbols into the interleaver memory andread data symbols from the interleaver memory into respective parts ofthe memory 410, 412 for successive sets of data symbols to implement anodd interleaving process only. Therefore, in correspondence withrepresentation shown in FIG. 9, FIG. 10 shows the mapping of theinterleaving process which is performed at the receiver and illustratedin FIG. 4 as an expanded view operating for both the first and secondhalves of the interleaving memory 410, 412. Thus a first set of datasymbols are written into a first part of the interleaver memory 410 in apermuted order defined in accordance with the addresses generated by theaddress generator 542 as illustrated by the order of writing in the datasymbols which provides a write sequence of 1, 3, 0, 2. As illustratedthe data symbols are then read out of the first part of the interleavermemory 410 in a sequential order thus recovering the original sequenceA, B, C, D.

Correspondingly, a second subsequent set of data symbols which arerecovered from a successive OFDM symbol are written into the second halfof the interleaver memory 412 in accordance with the addresses generatedby the address generator 542 in a permuted order and read out into theoutput data stream in a sequential order.

In one example the addresses generated for a first set of data symbolsto write into the first half of the interleaver memory 410 can be reusedto write a second subsequent set of data symbols into the interleavermemory 412. Correspondingly, the transmitter may also reuse addressesgenerated for one half of the interleaver for a first set of datasymbols for reading out a second set of data symbols which have beenwritten into the second half of the memory in sequential order.

Odd Interleaver with Offset

The performance of an interleaver, which uses two odd interleavers couldbe further improved by using a sequence of odd only interleavers ratherthan a single odd only interleaver, so that any bit of data input to theinterleave does not always modulate the same carrier in the OFDM symbol.

A sequence of odd only interleavers could be realised by either:

-   -   adding an offset to the interleaver address modulo the number of        data carriers, or    -   using a sequence of permutations in the interleaver

Adding an Offset

Adding an offset to the interleaver address modulo the number of datacarriers effectively shifts and wraps-round the OFDM symbol so that anybit of data input to the interleaver does not always modulate the samecarrier in the OFDM symbol. Thus the address generator, could optionallyinclude an offset generator, which generates an offset in an addressgenerated by the address generator on the output channel H(q).

The offset would change each symbol. For example, this offset couldprovide be a cyclic sequence. This cyclic sequence could be, forexample, of length 4 and could consist of, for example, prime numbers.For example, such a sequence could be:

0, 41, 97, 157

Furthermore, the offset may be a random sequence, which may be generatedby another address generator from a similar OFDM symbol interleaver ormay be generated by some other means.

Using a Sequence of Permutations

As shown in FIG. 5, a control line 111 extends from the control unit ofthe address generator to the permutation circuit. As mentioned above, inone example the address generator can apply a different permutation codefrom a set of permutation codes for successive OFDM symbols. Using asequence of permutations in the interleaver address generator reduces alikelihood that any bit of data input to the interleaver does not alwaysmodulate the same sub-carrier in the OFDM symbol.

For example, this could be a cyclic sequence, so that a differentpermutation code in a set of permutation codes in a sequence is used forsuccessive OFDM symbols and then repeated. This cyclic sequence couldbe, for example, of length two or four. For the example of the 4K symbolinterleaver a sequence of two permutation codes which are cycled throughper OFDM symbol could be for example:

7 10 5 8 1 2 4 9 0 3 6 **

6 2 7 10 8 0 3 4 1 9 5

Whereas a sequence of four permutation codes could be:

7 10 5 8 1 2 4 9 0 3 6 **

6 2 7 10 8 0 3 4 1 9 5

9 5 4 2 3 10 1 0 6 8 7

1 4 10 3 9 7 2 6 5 0 8

The switching of one permutation code to another could be effected inresponse to a change in the Odd/Even signal indicated on the controlchannel 108. In response the control unit 224 changes the permutationcode in the permutation code circuit 210 via the control line.

For the example of a 1k symbol interleaver, two permutation codes couldbe:

4 3 2 1 0 5 6 7 8

3 2 5 0 1 4 7 8 6

whereas four permutation codes could be:

4 3 2 1 0 5 6 7 8

3 2 5 0 1 4 7 8 6

7 5 3 8 2 6 1 4 0

1 6 8 2 5 3 4 0 7

Other combinations of sequences may be possible for 2k, 4k and 16kcarrier modes or indeed 0.5 k carrier mode. For example, the followingpermutation codes for each of the 0.5 k, 2k, 4k and 16k provide goodde-correlation of symbols and can be used cyclically to generate theoffset to the address generated by an address generator for each of therespective modes:

2k Mode:

0 7 5 1 8 2 6 9 3 4 *

4 8 3 2 9 0 1 5 6 7

8 3 9 0 2 1 5 7 4 6

7 0 4 8 3 6 9 1 5 2

8k Mode:

5 11 3 0 10 8 6 9 2 4 1 7 *

8 10 7 6 0 5 2 1 3 9 4 11

11 3 6 9 2 7 4 1 0 5 10 8

10 8 1 7 5 6 0 11 4 2 9 3

16k Mode:

8 4 3 2 0 11 1 5 12 10 6 7 9

7 9 5 3 11 1 4 0 2 12 10 8 6

6 11 7 5 2 3 0 1 10 8 12 9 4

5 12 9 0 3 10 2 4 6 7 8 11 1

For the permutation codes indicated above, the first two could be usedin a two sequence cycle, whereas all four could be used for a foursequence cycle. In addition, some further sequences of four permutationcodes, which are cycled through to provide the offset in an addressgenerator to produce a good de-correlation in the interleaved symbols(some are common to the above) are provided below:

0.5k Mode:

3 7 4 6 1 2 0 5

4 2 5 7 3 0 1 6

5 3 6 0 4 1 2 7

6 1 0 5 2 7 4 3

2k Mode:

0 7 5 1 8 2 6 9 3 4 *

3 2 7 0 1 5 8 4 9 6

4 8 3 2 9 0 1 5 6 7

7 3 9 5 2 1 0 6 4 8

4k Mode:

7 10 5 8 1 2 4 9 0 3 6 **

6 2 7 10 8 0 3 4 1 9 5

10 3 4 1 2 7 0 6 8 5 9

0 8 9 5 10 4 6 3 2 1 7

8k Mode:

5 11 3 0 10 8 6 9 2 4 1 7 *

10 8 5 4 2 9 1 0 6 7 3 11

11 6 9 8 4 7 2 1 0 10 5 3

8 3 11 7 9 1 5 6 4 0 2 10

*these are the permutations in the DVB-T standard

**these are the permutations in the DVB-H standard

Examples of address generators, and corresponding interleavers, for the2k, 4k and 8k modes are disclosed in European patent application number04251667.4, which corresponds to U.S. Patent Application No.2008/298487, the contents of which are incorporated herein by reference.An address generator for the 0.5 k mode are disclosed in our co-pendingUK patent application number 0722553.5, which corresponds to UKpublished application number GB 2454722. Various modifications may bemade to the embodiments described above without departing from the scopeof the present invention. In particular, the example representation ofthe generator polynomial and the permutation order which have been usedto represent aspects of the invention are not intended to be limitingand extend to equivalent forms of the generator polynomial and thepermutation order.

As will be appreciated the transmitter and receiver shown in FIGS. 1 and6 respectively are provided as illustrations only and are not intendedto be limiting. For example, it will be appreciated that the position ofthe symbol interleaver and the de-interleaver with respect, for exampleto the bit interleaver and the mapper can be changed. As will beappreciated the effect of the interleaver and de-interleaver isun-changed by its relative position, although the interleaver may beinterleaving I/Q symbols instead of v-bit vectors. A correspondingchange may be made in the receiver. Accordingly the interleaver andde-interleaver may be operating on different data types, and may bepositioned differently to the position described in the exampleembodiments.

According to one implementation of a receiver, a data processingapparatus is provided to map symbols received from a predeterminednumber of sub-carrier signals of an Orthogonal Frequency DivisionMultiplexed (OFDM) symbol into an output symbol stream.

As explained above the permutation codes and generator polynomial of theinterleaver, which has been described with reference to animplementation of a particular mode, can equally be applied to othermodes, by changing the predetermined maximum allowed address inaccordance with the number of sub-carriers for that mode.

As mentioned above, embodiments of the present invention findapplication with DVB standards such as DVB-T, DVB-T2 (published as EN302755) and DVB-H (published as ETSI EN 302 304 V1.1.1 (2004-11)), whichare incorporated herein by reference. For example embodiments of thepresent invention may be used in a transmitter or receiver operating inaccordance with the DVB-H standard, in hand-held mobile terminals. Themobile terminals may be integrated with mobile telephones (whethersecond, third or higher generation) or Personal Digital Assistants orTablet PCs for example. Such mobile terminals may be capable ofreceiving DVB-H or DVB-T/T2 compatible signals inside buildings or onthe move in for example cars or trains, even at high speeds. The mobileterminals may be, for example, powered by batteries, mains electricityor low voltage DC supply or powered from a car battery. Services thatmay be provided by DVB-H may include voice, messaging, internetbrowsing, radio, still and/or moving video images, television services,interactive services, video or near-video on demand and option. Theservices might operate in combination with one another. In otherexamples embodiments of the present invention finds application with theDVB-T2 standard as specified in accordance with ETSI standard EN 302755. In other examples embodiments of the present invention findapplication with the cable transmission standard known as DVB-C2.However, it will be appreciated that the present invention is notlimited to application with DVB and may be extended to other standardsfor transmission or reception, both fixed and mobile.

1. A data processing apparatus operable to map input symbols to becommunicated onto a predetermined number of sub-carrier signals ofOrthogonal Frequency Division Multiplexed (OFDM) symbols, the dataprocessing apparatus comprising an interleaver operable to read-into amemory the predetermined number of data symbols for mapping onto theOFDM sub-carrier signals, and to read-out of the memory the data symbolsfor the OFDM sub-carriers to effect the mapping, the read-out being in adifferent order than the read-in, the order being determined from a setof addresses, with the effect that the data symbols are interleaved onthe sub-carrier signals, an address generator operable to generate theset of addresses, an address being generated for each of the inputsymbols for mapping the input data symbol on to one of the sub-carriersignals, the address generator comprising a linear feedback shiftregister including a predetermined number of register stages and beingoperable to generate a pseudo-random bit sequence in accordance with agenerator polynomial, a permutation circuit operable to receive thecontent of the shift register stages and to permute the order of thebits present in the register stages in accordance with a permutationcode to form an address, and a control unit operable in combination withan address check circuit to re-generate an address when a generatedaddress exceeds a predetermined maximum valid address, wherein thepredetermined maximum valid address is approximately four thousand, thelinear feedback shift register has eleven register stages with agenerator polynomial for the linear feedback shift register ofR_(i)′[10]=R_(i-1)′[0]⊕R_(i-1)′[2], and the permutation code forms, withan additional bit, a twelve bit address, characterised in that thepermutation circuit is arranged to change the permutation code, whichpermutes the order of the bits of the register stages to form theaddresses from one OFDM symbol to another.
 2. A data processingapparatus as claimed in claim 1, wherein the permutation circuit isoperable to cycle through a sequence of different permutation codes forsuccessive OFDM symbols.
 3. A data processing apparatus as claimed inclaim 2, wherein one of the sequence of different permutation codesforms the twelve bit address R_(i)[n] for the i-th data symbol from thebit present in the n-th register stage R_(i)′[n] in accordance with thepermutation code defined by the table: R_(i)′[n] for n = 10 9 8 7 6 5 43 2 1 0 R_(i)[n] for n = 7 10 5 8 1 2 4 9 0 3 6


4. A data processing apparatus as claimed in claim 2, wherein thesequence of permutation codes comprises two permutation codes, whichare: R_(i)′[n] for n = 10 9 8 7 6 5 4 3 2 1 0 R_(i)[n] for n = 7 10 5 81 2 4 9 0 3 6

and R_(i)′[n] for n = 10 9 8 7 6 5 4 3 2 1 0 R_(i)[n] for n = 6 2 7 10 80 3 4 1 9 5


5. A data processing apparatus as claimed in claim 1, wherein thepredetermined maximum valid address is a value substantially between twothousand and four thousand and ninety six.
 6. A data processingapparatus as claimed in claim 5, wherein the OFDM symbol includes pilotsub-carriers, which are arranged to carry known symbols, and thepredetermined maximum valid address depends on a number of the pilotsub-carrier symbols present in the OFDM symbol.
 7. A data processingapparatus as claimed in claim 1, wherein the approximately four thousandsub-carriers is provided by one of a plurality of operating modes inwhich the approximately four thousand sub-carriers provides half or lessthan half a maximum number of sub-carriers in the OFDM symbols of any ofthe operating modes, and the input data symbols include first sets ofinput data symbols for mapping onto first OFDM symbols and second setsof input data symbols for mapping onto second OFDM symbols, and the dataprocessing apparatus is operable to interleave the input data symbolsfrom both first and second sets in accordance with an odd interleavingprocess, the odd interleaving process including writing the first setsof input data symbols into a first part of the interleaver memory inaccordance with a sequential order of the first sets of input datasymbols, reading out the first sets of input data symbols from the firstpart of the interleaver memory on to the sub-carrier signals of thefirst OFDM symbols in accordance with an order defined by the set ofaddresses generated with one of the permutation codes of the sequence,writing the second set of input data symbols into a second part of theinterleaver memory in accordance with a sequential order of the secondsets of input data symbols, and reading out the second sets of inputdata symbols from the second part of the interleaver memory on to thesub-carrier signals of the second OFDM symbols in accordance with anorder defined by the set of addresses generated with another of thepermutation codes of the sequence.
 8. A transmitter for transmittinginput data symbols using Orthogonal Frequency Division Multiplexing(OFDM), the transmitter including a data processing apparatus formapping the input data symbols on a predetermined number of sub-carriersignals of OFDM symbols, the data processing apparatus comprising aninterleaver operable to read-into a memory the predetermined number ofdata symbols for mapping onto the OFDM sub-carrier signals, and toread-out of the memory the data symbols for the OFDM sub-carriers toeffect the mapping, the read-out being in a different order than theread-in, the order being determined from a set of addresses, with theeffect that the data symbols are interleaved on the sub-carrier signals,an address generator operable to generate the set of addresses, anaddress being generated for each of the input symbols for mapping on toone of the sub-carrier signals, the address generator comprising alinear feedback shift register including a predetermined number ofregister stages and being operable to generate a pseudo-random bitsequence in accordance with a generator polynomial, a permutationcircuit operable to receive the content of the shift register stages andto permute the order of the bits present in the register stages inaccordance with a permutation code to form an address, and a controlunit operable in combination with an address check circuit tore-generate an address when a generated address exceeds a predeterminedmaximum valid address, wherein the predetermined maximum valid addressis approximately four thousand, the linear feedback shift register haseleven register stages with a generator polynomial for the linearfeedback shift register of R_(i)′[10]=R_(i-1)′[0]⊕R_(i-1)′[2], and thepermutation code forms, with an additional bit, a twelve bit address,characterised in that the permutation circuit is arranged to change thepermutation code, which permutes the order of the bits of the registerstages to form the addresses from one OFDM symbol to another.
 9. Atransmitter as claimed in claim 8, wherein the transmitter is operableto transmit data in accordance with a Digital Video Broadcastingstandard such as the Digital Video Broadcasting-Terrestrial, DigitalVideo Broadcasting-Handheld standard or the Digital VideoBroadcasting-Terrestrial2 standard.
 10. A method of mapping inputsymbols to be communicated onto a predetermined number of sub-carriersignals of an Orthogonal Frequency Division Multiplexed (OFDM) symbols,the method comprising reading-into a memory the predetermined number ofdata symbols for mapping onto the OFDM sub-carrier signals, reading-outof the memory the data symbols for the OFDM sub-carriers to effect themapping, the read-out being in a different order than the read-in, theorder being determined from a set of addresses, with the effect that thedata symbols are interleaved on the sub-carrier signals, generating theset of addresses, an address being generated for each of the inputsymbols for mapping the input data symbol on to one of the sub-carriersignals, the generating the set of addresses comprising using a linearfeedback shift register including a predetermined number of registerstages to generate a pseudo-random bit sequence in accordance with agenerator polynomial, using a permutation circuit operable to receivethe content of the shift register stages to permute the order of thebits present in the register stages in accordance with a permutationcode to form an address, and re-generating an address when a generatedaddress exceeds a predetermined maximum valid address, wherein thepredetermined maximum valid address is approximately four thousand, thelinear feedback shift register has eleven register stages with agenerator polynomial for the linear feedback shift register ofR_(i)′[10]=R_(i-1)′[0]⊕R_(i-1)′[2], and the permutation code forms, withan additional bit, a twelve bit address, characterised by changing thepermutation code, which permutes the order of the bits of the registerstages to form the set of addresses from one OFDM symbol to another. 11.A method as claimed in claim 10, wherein the changing the permutationcode includes cycling through a sequence of different permutation codesfor successive OFDM symbols.
 12. A method as claimed in claim 11,wherein one of the sequence of different permutation codes forms thetwelve bit address R_(i)[n] for the i-th data symbol from the bitpresent in the n-th register stage R_(i)′[n] in accordance with thepermutation code is defined by the table: R_(i)′[n] for n = 10 9 8 7 6 54 3 2 1 0 R_(i)[n] for n = 7 10 5 8 1 2 4 9 0 3 6


13. A method as claimed in claim 11, wherein the sequence of permutationcodes comprises two permutation codes, which are: R_(i)′[n] for n = 10 98 7 6 5 4 3 2 1 0 R_(i)[n] for n = 7 10 5 8 1 2 4 9 0 3 6

and R_(i)′[n] for n = 10 9 8 7 6 5 4 3 2 1 0 R_(i)[n] for n = 6 2 7 10 80 3 4 1 9 5


14. A method as claimed in claim 10, wherein the predetermined maximumvalid address is a value substantially between two thousand and fourthousand and ninety six.
 15. A method as claimed in claim 14, whereinthe OFDM symbol includes pilot sub-carriers, which are arranged to carryknown symbols, and the predetermined maximum valid address depends on anumber of the pilot sub-carrier symbols present in the OFDM symbol. 16.A method as claimed in claim 10, wherein the approximately four thousandsub-carriers is provided by one of a plurality of operating modes inwhich the approximately four thousand sub-carriers provide half or lessthan half a maximum number of sub-carriers in the OFDM symbols of any ofthe operating modes, the method comprising dividing the input datasymbols include first sets of input data symbols for mapping onto firstOFDM symbols and second sets of input data symbols for mapping ontosecond OFDM symbols, and interleaving the input data symbols from bothfirst and second sets in accordance with an odd interleaving processcomprising writing the first sets of input data symbols into a firstpart of the interleaver memory in accordance with a sequential order ofthe first sets of input data symbols, reading out the first sets ofinput data symbols from the first part of the interleaver memory on tothe sub-carrier signals of the first OFDM symbols in accordance with anorder defined by the set of addresses generated with one of thepermutation codes of the sequence, writing the second set of input datasymbols into a second part of the interleaver memory in accordance witha sequential order of the second sets of input data symbols, and readingout the second sets of input data symbols from the second part of theinterleaver memory on to the sub-carrier signals of the second OFDMsymbols in accordance with an order defined by the set of addressesgenerated with another of the permutation codes of the sequence.
 17. Amethod of transmitting data symbols via a predetermined number ofsub-carrier signals of an Orthogonal Frequency Division Multiplexed(OFDM) symbol, the method comprising reading-into a memory thepredetermined number of data symbols for mapping onto the OFDMsub-carrier signals, reading-out of the memory the data symbols fortransmission on the OFDM sub-carriers to effect the mapping, theread-out being in a different order than the read-in, the order beingdetermined from a set of addresses, with the effect that the datasymbols are interleaved on the sub-carrier signals, generating the setof addresses, an address being generated for each of the input symbolsfor mapping on to one of the sub-carrier signals, the generating the setof addresses comprising using a linear feedback shift register includinga predetermined number of register stages to generate a pseudo-randombit sequence in accordance with a generator polynomial, using apermutation circuit operable to receive the content of the shiftregister stages to permute the order of the bits present in the registerstages in accordance with a permutation code to form an address, andre-generating an address when a generated address exceeds apredetermined maximum valid address, wherein the predetermined maximumvalid address is approximately four thousand, the linear feedback shiftregister has eleven register stages with a generator polynomial for thelinear feedback shift register of R_(i)′[10]=R_(i-1)′[0]⊕R_(i-1)′[2],and the permutation code forms, with an additional bit, a twelve bitaddress, characterised by changing the permutation code, which permutesthe order of the bits of the register stages to form the set ofaddresses from one OFDM symbol to another.
 18. A method of transmittingas claimed in claim 17, wherein the transmitting includes transmittingin accordance with a Digital Video Broadcasting standard such as theDigital Video Broadcasting-Terrestrial, Digital VideoBroadcasting-Handheld standard or the Digital VideoBroadcasting-Terrestrial2 standard.
 19. An address generator for usewith transmission of data symbols interleaved onto sub-carriers of anOrthogonal Frequency Division Multiplexed symbol, the address generatorbeing operable to generate a set of addresses, each address beinggenerated for each of the data symbols for mapping the data symbols onto one of the sub-carrier signals, the address generator comprising alinear feedback shift register including a predetermined number ofregister stages and being operable to generate a pseudo-random bitsequence in accordance with a generator polynomial, a permutationcircuit operable to receive the content of the shift register stages andto permute the order of the bits present in the register stages inaccordance with a permutation code to form an address, and a controlunit operable in combination with an address check circuit tore-generate an address when a generated address exceeds a predeterminedmaximum valid address, wherein the predetermined maximum valid addressis approximately four thousand, the linear feedback shift register haseleven register stages with a generator polynomial for the linearfeedback shift register of R_(i)′[10]=R_(i-1)′[0]⊕R_(i-1)′[2], and thepermutation order forms, with an additional bit, a twelve bit address,characterised in that the permutation circuit is arranged to change thepermutation code, which permutes the order of the bits of the registerstages to form the set of addresses from one OFDM symbol to another. 20.An address generator as claimed in claim 19, wherein the permutationcircuit is operable to cycle through a sequence of different permutationcodes for successive OFDM symbols.
 21. An address generator as claimedin claim 20, wherein one of the sequence of different permutation codesforms the twelve bit address R_(i)[n] for the i-th data symbol from thebit present in the n-th register stage R_(i)′[n] in accordance with thepermutation code is defined by the table: R_(i)′[n] for n = 10 9 8 7 6 54 3 2 1 0 R_(i)[n] for n = 7 10 5 8 1 2 4 9 0 3 6


22. An address generator as claimed in claim 20, wherein the sequence ofpermutation codes comprises two permutation codes, which are: R_(i)′[n]for n = 10 9 8 7 6 5 4 3 2 1 0 R_(i)[n] for n = 7 10 5 8 1 2 4 9 0 3 6

and R_(i)′[n] for n = 10 9 8 7 6 5 4 3 2 1 0 R_(i)[n] for n = 6 2 7 10 80 3 4 1 9 5


23. A data processing apparatus operable to map input data symbols to becommunicated onto a predetermined number of sub-carrier signals ofOrthogonal Frequency Division Multiplexed OFDM symbols, thepredetermined number of sub-carrier signals being determined inaccordance with one of a plurality of operating modes and the input datasymbols including first sets of input data symbols for mapping ontofirst OFDM symbols and second sets of input data symbols for mappingonto second OFDM symbols, the data processing apparatus comprising aninterleaver operable to read-into a memory the predetermined number ofdata symbols for mapping onto the OFDM sub-carrier signals, and toread-out of the memory the data symbols for the OFDM sub-carriers toeffect the mapping, the read-out being in a different order than theread-in, the order being determined from a set of addresses, with theeffect that the data symbols are interleaved on the sub-carrier signals,an address generator operable to generate the set of addresses, anaddress being generated for each of the input symbols for mapping theinput data symbols onto one of the sub-carrier signals, the addressgenerator comprising a linear feedback shift register including apredetermined number of register stages and being operable to generate apseudo-random bit sequence in accordance with a generator polynomial, apermutation circuit operable to receive the content of the shiftregister stages and to permute the order of the bits present in theregister stages in accordance with a permutation code to form an addressof one of the OFDM sub-carriers, and a control unit operable incombination with an address check circuit to re-generate an address whena generated address exceeds a predetermined maximum valid address,wherein one of a plurality of operating modes provides approximatelyfour thousand sub-carriers per OFDM symbol, the approximately fourthousand sub-carriers providing half or less than half a maximum numberof sub-carriers in the OFDM symbols of any of the operating modes, thepredetermined maximum valid address is approximately four thousand, thelinear feedback shift register has eleven register stages with agenerator polynomial for the linear feedback shift register ofR_(i)′[10]=R_(i-1)′[0]⊕R_(i-1)′[2], and the permutation code forms, withan additional bit, a twelve bit address, and the data processingapparatus is adapted to interleave the input data symbols from bothfirst and second sets in accordance with an odd interleaving process,the odd interleaving process including writing the first sets of inputdata symbols into a first part of the interleaver memory in accordancewith a sequential order of the first sets of input data symbols, readingout the first sets of input data symbols from the first part of theinterleaver memory on to the sub-carrier signals of the first OFDMsymbols in accordance with an order defined by the set of addresses,writing the second set of input data symbols into a second part of theinterleaver memory in accordance with a sequential order of the secondsets of input data symbols, and reading out the second sets of inputdata symbols from the second part of the interleaver memory on to thesub-carrier signals of the second OFDM symbols in accordance with anorder defined by the set of addresses.
 24. A data processing apparatusas claimed in claim 23, wherein the permutation code forms the twelvebit address R_(i)[n] for the i-th data symbol from the bit present inthe n-th register stage R_(i)′[n] in accordance with the permutationcode defined by the table: R_(i)′[n] for n = 10 9 8 7 6 5 4 3 2 1 0R_(i)[n] for n = 7 10 5 8 1 2 4 9 0 3 6


25. A method of mapping input data symbols to be communicated onto apredetermined number of sub-carrier signals of Orthogonal FrequencyDivision Multiplexed OFDM symbols, the predetermined number ofsub-carrier signals being determined in accordance with one of aplurality of operating modes and the input data symbols including firstsets of input data symbols for mapping onto first OFDM symbols andsecond sets of input data symbols for second OFDM symbols, the methodcomprising reading-into a memory the predetermined number of datasymbols for mapping onto the OFDM sub-carrier signals, reading-out ofthe memory the data symbols for the OFDM sub-carriers to effect themapping, the read-out being in a different order than the read-in, theorder being determined from a set of addresses, with the effect that thedata symbols are interleaved on the sub-carrier signals, generating theset of addresses, an address being generated for each of the inputsymbols for mapping the input data symbol onto one of the sub-carriersignals, the generating the set of addresses comprising using a linearfeedback shift register including a predetermined number of registerstages to generate a pseudo-random bit sequence in accordance with agenerator polynomial, using a permutation circuit operable to receivethe content of the shift register stages to permute the order of thebits present in the register stages in accordance with a permutationcode to form an address, and re-generating an address when a generatedaddress exceeds a predetermined maximum valid address, wherein one of aplurality of operating modes provide approximately four thousandsub-carriers, the approximately four thousand sub-carriers providinghalf or less than half a maximum number of sub-carriers in the OFDMsymbols of any of the operating modes, the predetermined maximum validaddress is approximately four thousand, the linear feedback shiftregister has eleven register stages with a generator polynomial for thelinear feedback shift register of R_(i)′[10]=R_(i-1)′[0]⊕R_(i-1)′[2],and the permutation code forms, with an additional bit, a twelve bitaddress, and interleaving the input data symbols from both first andsecond sets in accordance with an odd interleaving process comprisingwriting the first sets of input data symbols into a first part of theinterleaver memory in accordance with a sequential order of the firstsets of input data symbols, reading out the first sets of input datasymbols from the first part of the interleaver memory on to thesub-carrier signals of the first OFDM symbols in accordance with anorder defined by the set of addresses, writing the second set of inputdata symbols into a second part of the interleaver memory in accordancewith a sequential order of the second sets of input data symbols, andreading out the second sets of input data symbols from the second partof the interleaver memory on to the sub-carrier signals of the secondOFDM symbols in accordance with an order defined by the set ofaddresses.
 26. A method as claimed in claim 25, wherein the permutationcode forms the twelve bit address R_(i)[n] for the i-th data symbol fromthe bit present in the n-th register stage R_(i)′[n] in accordance withthe permutation code is defined by the table: R_(i)′[n] for n = 10 9 8 76 5 4 3 2 1 0 R_(i)[n] for n = 7 10 5 8 1 2 4 9 0 3 6


27. A data processing apparatus operable to map input data symbols to becommunicated onto a predetermined number of sub-carrier signals ofOrthogonal Frequency Division Multiplexed OFDM symbols, thepredetermined number of sub-carrier signals being determined inaccordance with one of a plurality of operating modes and the input datasymbols including first sets of input data symbols for mapping ontofirst OFDM symbols and second sets of input data symbols for mappingonto second OFDM symbols, the data processing apparatus comprising aninterleaver operable to read-into a memory the predetermined number ofdata symbols for mapping onto the OFDM sub-carrier signals, and toread-out of the memory the data symbols for the OFDM sub-carriers toeffect the mapping, the read-out being in a different order than theread-in, the order being determined from a set of addresses, with theeffect that the data symbols are interleaved on the sub-carrier signals,an address generator operable to generate the set of addresses, anaddress being generated for each of the input symbols for mapping theinput data symbols onto one of the sub-carrier signals, the addressgenerator comprising a linear feedback shift register including apredetermined number of register stages and being operable to generate apseudo-random bit sequence in accordance with a generator polynomial, apermutation circuit operable to receive the content of the shiftregister stages and to permute the order of the bits present in theregister stages in accordance with a permutation code to form an addressof one of the OFDM sub-carriers, and a control unit operable incombination with an address check circuit to re-generate an address whena generated address exceeds a predetermined maximum valid address,wherein one of a plurality of operating modes provides approximatelyfour thousand sub-carriers per OFDM symbol, the approximately fourthousand sub-carriers providing half or less than half a maximum numberof sub-carriers in the OFDM symbols of any of the operating modes, thepredetermined maximum valid address is approximately four thousand, thelinear feedback shift register has eleven register stages with agenerator polynomial for the linear feedback shift register ofR_(i)′[10]=R_(i-1)′[0]⊕R_(i-1)′[2], and the permutation code forms, withan additional bit, a twelve bit address, and the data processingapparatus is adapted to interleave the input data symbols from bothfirst and second sets in accordance with an odd interleaving process,the odd interleaving process including writing the first sets of inputdata symbols into a first part of the interleaver memory in accordancewith a sequential order of the first sets of input data symbols, readingout the first sets of input data symbols from the first part of theinterleaver memory on to the sub-carrier signals of the first OFDMsymbols in accordance with an order defined by the set of addresses,writing the second set of input data symbols into a second part of theinterleaver memory in accordance with a sequential order of the secondsets of input data symbols, and reading out the second sets of inputdata symbols from the second part of the interleaver memory on to thesub-carrier signals of the second OFDM symbols in accordance with anorder defined by the set of addresses.
 28. A data processing apparatusas claimed in claim 27, wherein the permutation code forms the twelvebit address R_(i)[n] for the i-th data symbol from the bit present inthe n-th register stage R_(i)′[n] in accordance with the permutationcode defined by the table: R_(i)′[n] for n = 10 9 8 7 6 5 4 3 2 1 0R_(i)[n] for n = 7 10 5 8 1 2 4 9 0 3 6


29. A method of mapping input data symbols to be communicated onto apredetermined number of sub-carrier signals of Orthogonal FrequencyDivision Multiplexed OFDM symbols, the predetermined number ofsub-carrier signals being determined in accordance with one of aplurality of operating modes and the input data symbols including firstsets of input data symbols for mapping onto first OFDM symbols andsecond sets of input data symbols for second OFDM symbols, the methodcomprising reading-into a memory the predetermined number of datasymbols for mapping onto the OFDM sub-carrier signals, reading-out ofthe memory the data symbols for the OFDM sub-carriers to effect themapping, the read-out being in a different order than the read-in, theorder being determined from a set of addresses, with the effect that thedata symbols are interleaved on the sub-carrier signals, generating theset of addresses, an address being generated for each of the inputsymbols for mapping the input data symbol onto one of the sub-carriersignals, the generating the set of addresses comprising using a linearfeedback shift register including a predetermined number of registerstages to generate a pseudo-random bit sequence in accordance with agenerator polynomial, using a permutation circuit operable to receivethe content of the shift register stages to permute the order of thebits present in the register stages in accordance with a permutationcode to form an address, and re-generating an address when a generatedaddress exceeds a predetermined maximum valid address, wherein one of aplurality of operating modes provide approximately four thousandsub-carriers, the approximately four thousand sub-carriers providinghalf or less than half a maximum number of sub-carriers in the OFDMsymbols of any of the operating modes, the predetermined maximum validaddress is approximately four thousand, the linear feedback shiftregister has eleven register stages with a generator polynomial for thelinear feedback shift register of R_(i)′[10]=R_(i-1)′[0]⊕R_(i-1)′[2],and the permutation code forms, with an additional bit, a twelve bitaddress, and interleaving the input data symbols from both first andsecond sets in accordance with an odd interleaving process comprisingwriting the first sets of input data symbols into a first part of theinterleaver memory in accordance with a sequential order of the firstsets of input data symbols, reading out the first sets of input datasymbols from the first part of the interleaver memory on to thesub-carrier signals of the first OFDM symbols in accordance with anorder defined by the set of addresses, writing the second set of inputdata symbols into a second part of the interleaver memory in accordancewith a sequential order of the second sets of input data symbols, andreading out the second sets of input data symbols from the second partof the interleaver memory on to the sub-carrier signals of the secondOFDM symbols in accordance with an order defined by the set ofaddresses.
 30. A method as claimed in claim 29, wherein the permutationcode forms the twelve bit address R_(i)[n] for the i-th data symbol fromthe bit present in the n-th register stage R_(i)′[n] in accordance withthe permutation code is defined by the table: R_(i)′[n] for n = 10 9 8 76 5 4 3 2 1 0 R_(i)[n] for n = 7 10 5 8 1 2 4 9 0 3 6


31. A transmitter for transmitting input symbols using a predeterminednumber of sub-carrier signals of Orthogonal Frequency DivisionMultiplexed (OFDM) symbols, the transmitter comprising an interleaveroperable to read-into a memory the predetermined number of data symbolsfor mapping onto the OFDM sub-carrier signals, and to read-out of thememory the data symbols for the OFDM sub-carriers to effect the mapping,the read-out being in a different order than the read-in, the orderbeing determined from a set of addresses, with the effect that the datasymbols are interleaved on the sub-carrier signals, an address generatorconfigured to provide the set of addresses, an address being generatedfor each of the input symbols for mapping the input data symbols on toone of the sub-carrier signals, the address generator comprising amemory containing the set of addresses, the set of addresses having beengenerated and stored in the memory using a linear feedback shiftregister including a predetermined number of register stages and beingoperable to generate a pseudo-random bit sequence in accordance with agenerator polynomial, a permutation circuit operable to receive thecontent of the shift register stages and to permute the order of thebits present in the register stages in accordance with a permutationcode to form an address, and a control unit operable in combination withan address check circuit to re-generate an address when a generatedaddress exceeds a predetermined maximum valid address, wherein thepredetermined maximum valid address is approximately four thousand, thelinear feedback shift register has eleven register stages with agenerator polynomial for the linear feedback shift register ofR_(i)′[10]=R_(i-1)′[0]⊕R_(i-1)′[2], and the permutation code forms, withan additional bit, a twelve bit address, characterised in that thepermutation circuit is arranged to change the permutation code, whichpermutes the order of the bits of the register stages to form theaddresses from one OFDM symbol to another.
 32. A method of transmittinginput symbols using a predetermined number of sub-carrier signals of anOrthogonal Frequency Division Multiplexed (OFDM) symbols, the methodcomprising reading-into a memory the predetermined number of datasymbols for mapping onto the OFDM sub-carrier signals, reading-out ofthe memory the data symbols for the OFDM sub-carriers to effect themapping, the read-out being in a different order than the read-in, theorder being determined from a set of addresses, with the effect that thedata symbols are interleaved on the sub-carrier signals, providing theset of addresses, an address being provided for each of the inputsymbols for mapping the input data symbol on to one of the sub-carriersignals, the providing the set of addresses comprising reading the setof addresses from a memory containing the set of addresses, the set ofaddresses having been generated and stored in the memory by using alinear feedback shift register including a predetermined number ofregister stages to generate a pseudo-random bit sequence in accordancewith a generator polynomial, using a permutation circuit operable toreceive the content of the shift register stages to permute the order ofthe bits present in the register stages in accordance with a permutationcode to form an address, and re-generating an address when a generatedaddress exceeds a predetermined maximum valid address, wherein thepredetermined maximum valid address is approximately four thousand, thelinear feedback shift register has eleven register stages with agenerator polynomial for the linear feedback shift register ofR_(i)′[10]=R_(i-1)′[0]⊕R_(i-1)[2], and the permutation code forms, withan additional bit, a twelve bit address, characterised by changing thepermutation code, which permutes the order of the bits of the registerstages to form the set of addresses from one OFDM symbol to another. 33.A transmitter for mapping input data symbols onto a predetermined numberof sub-carrier signals of Orthogonal Frequency Division Multiplexed OFDMsymbols and transmitting the data symbols via the OFDM symbols, thepredetermined number of sub-carrier signals being determined inaccordance with one of a plurality of operating modes and the input datasymbols including first sets of input data symbols for mapping ontofirst OFDM symbols and second sets of input data symbols for mappingonto second OFDM symbols, the transmitter comprising an interleaveroperable to read-into a memory the predetermined number of data symbolsfor mapping onto the OFDM sub-carrier signals, and to read-out of thememory the data symbols for the OFDM sub-carriers to effect the mapping,the read-out being in a different order than the read-in, the orderbeing determined from a set of addresses, with the effect that the datasymbols are interleaved on the sub-carrier signals, an address generatorconfigured to provide the set of addresses, an address being generatedfor each of the input symbols for mapping the input data symbols on toone of the sub-carrier signals, the address generator comprising amemory containing the set of addresses, the set of addresses having beengenerated and stored in the memory using a linear feedback shiftregister including a predetermined number of register stages and beingoperable to generate a pseudo-random bit sequence in accordance with agenerator polynomial, a permutation circuit operable to receive thecontent of the shift register stages and to permute the order of thebits present in the register stages in accordance with a permutationcode to form an address of one of the OFDM sub-carriers, and a controlunit operable in combination with an address check circuit tore-generate an address when a generated address exceeds a predeterminedmaximum valid address, wherein one of a plurality of operating modesprovides approximately four thousand sub-carriers per OFDM symbol, theapproximately four thousand sub-carriers providing half or less thanhalf a maximum number of sub-carriers in the OFDM symbols of any of theoperating modes, the predetermined maximum valid address isapproximately four thousand, the linear feedback shift register haseleven register stages with a generator polynomial for the linearfeedback shift register of R_(i)′[10]=R_(i-1)′[0]⊕R_(i-1)′[2], and thepermutation code forms, with an additional bit, a twelve bit address,and the data processing apparatus is adapted to interleave the inputdata symbols from both first and second sets in accordance with an oddinterleaving process, the odd interleaving process including writing thefirst sets of input data symbols into a first part of the interleavermemory in accordance with a sequential order of the first sets of inputdata symbols, reading out the first sets of input data symbols from thefirst part of the interleaver memory on to the sub-carrier signals ofthe first OFDM symbols in accordance with an order defined by the set ofaddresses, writing the second set of input data symbols into a secondpart of the interleaver memory in accordance with a sequential order ofthe second sets of input data symbols, and reading out the second setsof input data symbols from the second part of the interleaver memory onto the sub-carrier signals of the second OFDM symbols in accordance withan order defined by the set of addresses.
 34. A method of transmittinginput data symbols using a predetermined number of sub-carrier signalsof Orthogonal Frequency Division Multiplexed OFDM symbols, thepredetermined number of sub-carrier signals being determined inaccordance with one of a plurality of operating modes and the input datasymbols including first sets of input data symbols for mapping ontofirst OFDM symbols and second sets of input data symbols for second OFDMsymbols, the method comprising reading-into a memory the predeterminednumber of data symbols for mapping onto the OFDM sub-carrier signals,reading-out of the memory the data symbols for the OFDM sub-carriers toeffect the mapping, the read-out being in a different order than theread-in, the order being determined from a set of addresses, with theeffect that the data symbols are interleaved on the sub-carrier signals,providing the set of addresses, an address being provided for each ofthe input symbols for mapping the input data symbol on to one of thesub-carrier signals, the providing the set of addresses comprisingreading the set of addresses from a memory containing the set ofaddresses, the set of addresses having been generated and stored in thememory by using a linear feedback shift register including apredetermined number of register stages to generate a pseudo-random bitsequence in accordance with a generator polynomial, using a permutationcircuit operable to receive the content of the shift register stages topermute the order of the bits present in the register stages inaccordance with a permutation code to form an address, and re-generatingan address when a generated address exceeds a predetermined maximumvalid address, wherein one of a plurality of operating modes provideapproximately four thousand sub-carriers, the approximately fourthousand sub-carriers providing half or less than half a maximum numberof sub-carriers in the OFDM symbols of any of the operating modes, thepredetermined maximum valid address is approximately four thousand, thelinear feedback shift register has eleven register stages with agenerator polynomial for the linear feedback shift register ofR_(i)′[10]=R_(i-1)′[0]⊕R_(i-1)′[2], and the permutation code forms, withan additional bit, a twelve bit address, and interleaving the input datasymbols from both first and second sets in accordance with an oddinterleaving process comprising writing the first sets of input datasymbols into a first part of the interleaver memory in accordance with asequential order of the first sets of input data symbols, reading outthe first sets of input data symbols from the first part of theinterleaver memory on to the sub-carrier signals of the first OFDMsymbols in accordance with an order defined by the set of addresses,writing the second set of input data symbols into a second part of theinterleaver memory in accordance with a sequential order of the secondsets of input data symbols, and reading out the second sets of inputdata symbols from the second part of the interleaver memory on to thesub-carrier signals of the second OFDM symbols in accordance with anorder defined by the set of addresses.